The present invention relates to a method of preparing a casting mold comprising at least one mold cavity adapted to receive molten metal directed to said cavity by at least one pouring gate opening out into a runner extending downstream from the pouring gate to said cavity, the method being implemented in an automatic casting machine and including a step of preparing two mold halves and a step of assembling the two mold halves on a substantially vertical parting face.
Automatic machines for casting molten metal are known, e.g. machines available on the market under the trademark xe2x80x9cDISAMATICxe2x80x9d(copyright).
In such a machine, molds are placed on a closed circuit conveyor whereby the molds pass successively through stations for preparing the two mold halves, for assembling the two halves on a substantially vertical parting face, for casting molten metal into the mold made up in this way, for solidifying the cast metal inside the mold, and for disassembling the mold to release the casting(s).
Numerous tests have been performed attempting to interpose a filter on the passage of the molten metal so as to filter the molten metal before it reaches the mold cavity.
When molding hollow pieces, it is necessary to install at least one core in the mold cavity. The operation of installing the core is relatively lengthy and difficult. It is easy at the same time to install a filter on the path of the molten metal in the runner which is generally disposed astride the two mold halves.
In contrast, when no core is necessary, the operations of preparing the two mold halves are very quick, so all attempts at installing a filter have given negative results, with the placing of a filter giving rise to more drawbacks in terms of disturbing the operating cycle of the automatic casting machine than advantages in terms of improving the internal structure of the cast metal. In other words, industry prefers to favor the productivity of the automatic casting machine over a significant percentage of pieces that are rejected for faults concerning metal quality.
There thus exists a need for a method of the above type enabling a filter to be inserted on the path of the cast metal with practically no reduction in the productivity of an automatic machine for casting molten metal.
According to the present invention, the method of the above-specified type is characterized in that it comprises the following steps:
providing a recess in a first half of the mold to form a pouring gate extending to the parting face;
providing a housing in the parting face, substantially parallel to said parting face and at the periphery of the recess, the top portion of the housing presenting a top opening, said housing having predetermined dimensions parallel to said parting face that are greater than those of said gate and having predetermined thickness in the direction perpendicular to said parting face;
providing a second recess in the second half of the mold having an opening in the parting face of dimensions that are smaller than the corresponding dimensions of the housing, the top portion of said recess being closed and said recess communicating downstream with a runner;
assembling together the two mold halves on the parting face; and
inserting a filter into the housing substantially vertically through the top opening thereof, which filter has dimensions that are perceptibly smaller than the corresponding predetermined dimensions of the housing and is adapted to filter molten metal so that molten metal poured into the gate passes through said filter and is then channeled by the second recess towards the runner.
The housing provided in the first mold half is upwardly open and enables a filter to be inserted easily and reliably in the top opening of said housing. This housing is closed at its periphery by the edges defining the opening of the recess in the second mold half. When the molten metal comes into the gate, this molten metal presses the filter against the second mold half. The filter is thus pressed in substantially leakproof manner against the edges of the opening of the second mold half, thereby obliging the molten metal to pass through the filter and be filtered prior to penetrating into the mold cavity. A passage is thus provided for the molten metal enabling a filter to be inserted across the passage without disturbing the operation of an automatic casting machine.
This insertion of the filter can be performed either during any mold waiting time, e.g. during the duration of several seconds required for casting the molten metal into a preceding mold, or else even while the mold is being moved.
The clearance between the walls of the recess and those of the filter is sufficient to eliminate any risk of the filter jamming while it is being inserted, and the vibration accompanying displacement can further facilitate such insertion of the filter.
Furthermore, the person skilled in the art has been dissuaded from putting a filter into place in a substantially vertical position without any means for preventing possible uplift of the filter under the effect of buoyancy pressure, given that the density of the filter is much less than that of the molten metal.
Experience has confirmed that from the beginning of molten metal being poured into the gate, the molten metal presses the filter against the edges of the second recess so that friction against said edges prevents any displacement of the filter.
In an advantageous version of the invention, the housing and the filter are given respective complementary outlines in cross-section on a plane parallel to the parting face, which outlines are shaped in such a manner as to center the filter automatically relative to the housing, e.g. outlines of trapezoidal shape with the small base downwards.
In a preferred embodiment, a filter is used comprising a series of at least two filter plates of refractory material defining a corresponding cavity between two adjacent plates, each of said plates having a series of filter holes allowing molten metal to pass through and be filtered, at least one of the cavities containing a molten metal treatment material in the form of a compressed, sintered, or molded slab, the slab of treatment material being of a shape such that when seen in the travel direction of the molten metal, it leaves at least one uncovered region having filter holes that are not obstructed by said slab in each of the filter plates so as to enable molten metal that has penetrated into the cavity to come into contact with the treatment material of the slab and flow around said slab and then through the filter holes of the second filter plate.
Such a filter is known, in particular form EP-A-0 578 517 and FR 98/01634 in the name of the Applicant.
The cavity provided between the two plates fills with molten metal substantially at the same time as the gate, thus weighing down the filter. This weighing down of the filter, combined with the fact that the molten metal present in the gate presses the filter against the edges of the second recess, ensures that there is no risk of the filter rising under the effect of buoyancy pressure.
In addition, the molten metal is not only filtered but is also treated under excellent conditions of reliability and uniformity of treatment.
In another aspect of the invention, a mold for casting molten metal of the present invention, comprises at least one mold cavity adapted to receive molten metal directed to said cavity by at least one pouring gate opening out into a runner extending downstream from the pouring gate to said cavity with a filter being interposed on the path of the molten metal, and is characterized in that said mold is prepared by implementing the method of the first aspect of the invention.
Other features and advantages of the present invention appear in the following detailed description.